Detection of instants at which the heart is quiescent, in other words, temporarily still within the normal cardiac cycle, is known within medical imaging and allows the presentation of acquired diagnostic data which is corrected for heart movement. In particular, ‘Application Guide, GYROSCAN NT VOLUME 2: Scan Methods’, section 5.9, Coronary Artery Imaging, describes the use of a trigger delay to capture images of the heart at a relatively quiet moment. ‘Magnetic Resonance Imaging, Physical Principles and Sequence Design’, by E. Mark Haacke, et al., Wiley-Liss, ISBN 0-471-35128-8, section 24.4.1, describes triggering data acquisition to the cardiac cycle. Together, these discloses indicate how techniques of magnetic resonance imaging in combination with ECG gating can be exploited to identify the instants in time at which the beating human heart is quiescent within the normal cycle of pulsation.
U.S. Pat. No. 3,483,860 discloses the use of doppler radar to measure the activity of the heart and in particular discloses that the output doppler signal can be subjected to mathematical differentiation to provide a signal representing the blood ejection rate.
Magnetic resonance imaging is expensive and the image data takes considerable time to acquire. Therefore magnetic resonance imaging is not suitable for regular monitoring of heart activity and in particular is not suitable for ambulatory monitoring. In addition it is not suitable for measurement of the quiescent periods of any other pulsating object in the body, for example an artery, because the reliance on ECG gating ties the application of this imaging technique to the heart.